U.S. patent number 6,645,301 [Application Number 09/925,187] was granted by the patent office on 2003-11-11 for ion source.
This patent grant is currently assigned to Saintech Pty Limited. Invention is credited to Wayne G Sainty.
United States Patent |
6,645,301 |
Sainty |
November 11, 2003 |
Ion source
Abstract
An ion source for use in ion assisted deposition of films, has
an ionization region, a gas supply supplying ionizable gas to the
ionization region, a gas excitation system causing ionization of
the gas, ion influencing means forming the ions into a current
directed at a target, and an ion source controller controlling the
ion source so as to intermittently produce the ion current.
Inventors: |
Sainty; Wayne G (Beecroft,
AU) |
Assignee: |
Saintech Pty Limited (New South
Wales, AU)
|
Family
ID: |
3825832 |
Appl.
No.: |
09/925,187 |
Filed: |
August 9, 2001 |
Current U.S.
Class: |
118/665;
118/723FI; 118/723R; 204/298.04; 204/298.11; 313/359.1; 313/361.1;
313/362.1; 313/363.1; 315/111.81; 315/111.91; 118/723MP |
Current CPC
Class: |
H01J
27/02 (20130101); H01J 37/08 (20130101); H01J
27/146 (20130101); C23C 14/54 (20130101); C23C
14/221 (20130101) |
Current International
Class: |
C23C
14/54 (20060101); C23C 14/22 (20060101); H01J
37/08 (20060101); B05C 011/00 (); C23C 016/00 ();
C23C 014/00 () |
Field of
Search: |
;204/298.04,298.14,298.08,298.16,298.06,298.34,298.36,298.11
;118/723,723R,723FI,723MP,665 ;156/345.39,345.4
;313/564,566,359.1,361.1,362.1,363.1 ;315/111.81,111.91 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
2 123 441 |
|
Feb 1984 |
|
GB |
|
96/22841 |
|
Aug 1996 |
|
WO |
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98/48073 |
|
Oct 1998 |
|
WO |
|
01/55475 |
|
Aug 2001 |
|
WO |
|
Other References
JP 09092199 abstract. .
JP 3222-095 A abstract. .
JP 11273894-A abstract..
|
Primary Examiner: VerSteeg; Steven H.
Attorney, Agent or Firm: Gordon & Jacobson, P.C.
Claims
I claim:
1. An ion source including: an ionzation region; a gas supply; a
gas excitation system; ion influencing means; and an ion source
controller; wherein said gas supply supplies an ionizable gas to
said ionization region; wherein said gas excitation system causes
ionization of gas in said ionization region; wherein said ion
influencing means forms ions produced in said ionization region
into an ion current substantially directed at a target; and wherein
said ion source controller controls said ion source so as to
intermittently produce said ion current.
2. An ion source according to claim 1 wherein said ion source
controller includes a signal generator producing a regular waveform
signal controlling the production of said ion current.
3. A thin film deposition system including: deposition apparatus;
and an ion source according to claim 1; wherein said deposition
apparatus ejects a stream of deposition material towards a target,
and wherein deposition of material onto the target is substantially
prevented whilst the target is subjected to said ion current.
4. A system according to claim 3 further including a shutter member
that substantially blocks said stream of deposition material whilst
the target is subjected to said ion current.
5. A system according to claim 4 wherein said shutter member is
controlled by said ion source controller.
6. A system according to claim 5 wherein said ion source controller
includes a signal generator producing a pulse waveform signal that
controls said ion source and said shutter member.
7. A system according to claim 3 further including a pressure
monitor wherein said pressure monitor measures the pressure of said
system and wherein deposition of material onto the target
recommences in response to a pressure measurement below a
predetermined level.
8. A thin film deposition system including: deposition apparatus;
an ion source according to claim 1; and a deposition monitors,
wherein said deposition apparatus ejects a stream of material
toward a target, wherein said deposition monitor monitors the
increase in thickness of material deposited on said target, wherein
said deposition monitor triggers said ion source to commence
production of said ion current in response to a measured increase
in deposited material above a predetermined level, and wherein said
ion source controller controls said ion source to produce said ion
current for a predetermined duration.
9. A system according to claim 8 wherein said predetermined level
is between 5 and 30 nm.
10. A system according to claim 8 wherein said predetermined
duration is between 0.5 and 5 seconds.
11. An ion source including: an ionization region; a gas supply; a
cathode; a cathode emission controller; an anode an electric
potential generator; and ion influencing means; wherein said gas
supply supplies an ionizable gas to said ionization region; wherein
said cathode is disposed at one end of said ionization region;
wherein said anode is disposed at an opposite longitudinal end of
said ionization region; wherein said cathode emission controller
causes said cathode to emit electrons; wherein said electric
potential generator generates an electric potential between said
cathode and said anode; wherein said generated electric potential
causes electrons emitted by said cathode to accelerate in the
direction of said anode; wherein electrons moving toward said anode
bombard said ionization gas to produce ions; wherein said ion
influencing means forms ions produced in said ionization region
into an ion current substantially directed at a target; and wherein
said cathode emission controller causes intermittent emission of
electrons from said cathode such that said ion source produces an
intermittent ion current.
12. An ion source according to claim 11 wherein said cathode
emission controller generates a waveform current signal that is
provided to said cathode to stimulate emission of electrons from
said cathode.
13. An ion source according to claim 11 wherein said cathode
emission controller generates a DC current signal that is provided
to said cathode to stimulate emission of electrons from said
cathode, wherein said DC current signal includes a continuous base
current signal below a threshold current required for electron
emission from said cathode and an intermittent pulse current signal
superimposed on said base current signal and wherein the
combination of said base current and said pulse current is above
the threshold current required for electron emission from said
cathode.
14. An ion source according to claim 13 wherein the pulse current
signal has a duty cycle between 5% and 20%.
15. An ion source according to claim 13 wherein the total current
to said cathode is above the threshold for electron emission for 5
to 20% of the total signal period.
16. An ion source including: an ionization region; a gas supply; a
gas excitation system; ion influencing means; and a gas flow
controller; wherein said gas supply supplies an ionizable gas to
said ionization region; wherein said gas excitation system causes
ionization of gas in said ionization region; wherein said ion
influencing means forms ions produced in said ionization region
into an ion current substantially directed at a target; and wherein
said gas flow controller controls the flow of gas into said
ionization region so as to intermittently produce said ion
current.
17. An ion source according to claim 16 wherein the commencement of
said ion current is in response to an external trigger received by
said ion source controller, said ion source controller further
including a timer for controlling the duration of said ion
current.
18. An ion source according to claim 16 wherein said gas supply
includes a gas line, an outlet in said gas line to said ionization
region and a valve disposed in said gas line to control the flow of
gas to said outlet, wherein said gas flow controller includes a
signal generator and wherein said signal generator generates a
signal to control the opening and closing of said valve.
19. An ion source according to claim 18 wherein said valve is
substantially adjacent said outlet.
20. An ion source according to claim 18 wherein said signal is a
regular pulse waveform signal.
21. An ion source according to claim 20 wherein said regular pulse
waveform signal has a duty cycle of between 5% and 30%.
22. An ion source according to claim 18 wherein said valve is
electrically controlled by a signal from said signal generator.
23. An ion source according to claim 18 wherein said valve includes
a valve seat formed in said gas line, an armature disposed in said
gas line, and a coil disposed around said gas line, wherein said
armature is adapted to sealingly engage said valve seat to prevent
the flow of gas through said gas line, wherein said coil is charged
by a signal from said signal generator, and wherein said armature
disengages said valve seat in response to a change in the charging
state of said coil.
24. An ion source according to claim 23 wherein said armature is
biased towards a position in which said armature engages said valve
seat.
25. An ion source according to claim 24 further including a magnet,
wherein the magnetic field from said magnet biases said armature to
engage said valve seat.
26. An ion source according to claim 16 wherein said gas supply
includes one or more gas injectors, wherein said gas injectors
inject a measured amount of gas into said ionization region and
wherein the injection of gas from said gas injectors is controlled
by said gas flow controller.
27. A thin film deposition system including deposition apparatus
and an ion source according to claim 16 wherein said deposition
apparatus ejects a stream of deposition material toward said
target.
28. An ion source including: an ionization region; a gas supply; a
cathode; a cathode emission controller; an anode; a magnetic field
generator; and an electric potential generator; wherein said gas
supply supplies an ionizable gas to said ionization region; wherein
said cathode is disposed at one end of said ionization region;
wherein said anode is disposed at an opposite longitudinal end of
said ionization region; wherein a longitudinal axis lies
substantially between said anode and said cathode; wherein said
magnetic field generator generates a magnetic field the axis of
which lies substantially parallel to said longitudinal axis;
wherein said cathode emission controller causes said cathode to
emit electrons; wherein said electric potential generator generates
an electric potential between said cathode and said anode; wherein
said generated electric potential causes electrons emitted by said
cathode to accelerate in the direction of said anode; wherein
electrons moving toward said anode bombard said ionizable gas to
produce ions; wherein said electric potential and said magnetic
field act together to form ions produced in said ionization region
into an ion current substantially directed at a target; and wherein
said electric potential is generated intermittently such that said
ion source produces an intermittent ion current.
29. An ion source according to claim 28 wherein said electric
potential generator provides an electric potential to said anode
that causes electrons emitted by said cathode to accelerate in the
direction of said anode.
30. An ion source according to claim 28 wherein said gas supply
includes a gas line passing substantially through said anode, said
gas line terminating in an outlet in said ionization region;
wherein said ion source includes a longitudinal axis extending
generally between said anode and said cathode; wherein said anode
is substantially centered on said longitudinal axis; wherein said
outlet is disposed on said longitudinal axis of said ion source;
and wherein said outlet includes a plurality of apertures.
31. An ion source according to claim 28 wherein said ion source
includes a longitudinal axis extending generally between said anode
and said cathode, said ion source further including a magnetic
field generator, wherein said magnetic field generator generates a
magnetic field the axis of which lies substantially parallel to
said longitudinal axis.
32. A control system for controlling an ion-assisted deposition
process including: a deposition monitor; a pressure monitor; an ion
source controller; and a deposition controller; wherein said
deposition monitor monitors the increase in thickness of deposition
material on a substrate, wherein said pressure monitor measures the
pressure within a vacuum chamber in which said ion assisted
deposition process occurs, wherein a first control signal is
generated in response to a measurement by said deposition monitor
of an increase in thickness of deposited material on said substrate
above a predetermined amount, wherein said deposition controller
causes deposition of material onto said substrate to cease in
response to said first control signal, wherein said ion source
controller causes an ion source to produce an ion current directed
at said substrate for a pre-determined duration in response to said
first control signal, wherein said ion current is produced from an
ionizable gas wherein after the expiration of said predetermined
duration a second control signal is generated in response to a
measurement of pressure by said pressure monitor below a
predetermined pressure, and wherein said deposition controller
causes the deposition of material on said substrate to recommence
in response to said second control signal.
Description
BACKGROUND OF THE INVENTION
This invention relates to ion sources used in Ion Assisted
Deposition (IAD) of films, in particular optical quality films, and
to methods of operating such ion sources.
Ion sources had their origins in space propulsion but more recently
have found use in industrial processes such as IAD of thin film
coatings. In an IAD process, an ion beam from an ion source is
directed toward a target substrate to cause densification of the
coating material as it is deposited. The process occurs within an
evacuated chamber of pressure of the order 10.sup.-2 Pa or
less.
The benefits that result from ion assistance, during growth, of
almost any optical material is well understood and is today widely
practiced. In general, ion bombardment provides close to bulk
density of the film resulting in dramatic improvements in
durability and performance. However, for many classes of materials
this benefit is accompanied by an undesirable modification of
optical properties observed as an increasing absorption coefficient
(k) and variability in refractive index (n). For many classes of
materials, this problem results from incompatibility between the
ion species and depositing material.
Argon and oxygen are the two most predominant species of ions used
in IAD processes. The high momentum of Ar+ provides high packing
density, although usually leads to a reduction of metal oxides and
fluorine depletion of most metal fluorides. This results in
metal-rich films with a subsequent increase in optical
absorption.
The use of O+ is well suited to the IAD of metal oxides such as
titania, silica etc. With the correct choice of energy and ion
current density, O+ IAD can provide fully densified and low-stress
films. Problems arise however where the very chemically active
oxygen ions displace fluorine atoms from depositing molecules
immediately prior to their incorporation in the film. This leads to
the growth of oxy-fluorides with subsequent deterioration of
optical properties. The extent to which this occurs depends on
factors such as ion energy and current.
SUMMARY OF THE INVENTION
In a first aspect, the invention resides in an ion source
including: an ionization region; a gas supply; a gas excitation
system; ion influencing means; and an ion source controller;
wherein said gas supply supplies an ionizable gas to said
ionization region;
wherein said gas excitation system causes ionization of gas in said
ionization region;
wherein said ion influencing means forms ions produced in said
ionization region into an ion current substantially directed at a
target;
and wherein said ion source controller controls said ion source so
as to intermittently produce said ion current.
In a first embodiment, gas is intermittently introduced into the
ionization region.
In a second embodiment, the flow of electrons into the ionization
region is made intermittent.
In a further embodiment the ion source of the present invention is
combined with a film deposition apparatus, the combined apparatus
including a deposition control system that prevents deposition of
new material onto the target substrate while the ion current is
directed towards the target.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the invention will become
apparent to the skilled reader from the following description of
preferred embodiments made with reference to the accompanying
Figures in which:
FIG. 1 is a partial cross-sectional elevation of the ion source
according to the invention,
FIG. 2 is a plan view of the ion source in FIG. 1,
FIG. 3 shows an example of a cathode filament waveform signal,
FIG. 4 is a side view of a gas delivery system with a generic
control valve adjacent the outlet,
FIG. 5 is shows an example of an outlet control valve, and
FIG. 6 is a schematic of an ion source combined with IAD deposition
apparatus.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In a typical ion source, electrons are drawn from a cathode
filament toward an anode through an ionizable gas. Collisions
between the gas molecules and energetic electrons create a source
of positive ions by inducing a plasma. In one type of ion source
known as a gridless ion source, a magnetic field is applied across
the plasma to shape the ions accelerated from the ion source into
an ion beam. In a specific type of gridless ion source, known as an
end-Hall effect ion source, the axis of the magnetic field is
aligned with the electric potential between the cathode and the
anode. The interaction of the magnetic and electric fields causes
the charged particles to approximately follow the magnetic field
lines. The anode in these devices is typically annular having an
outwardly inclined inner diameter with the bulk of the plasma
forming within the confines of the anode walls.
A specific ion source is described below but it is to be understood
that the description is for illustrative purposes only. The present
invention can be suitably adapted for use with any one of several
known ion sources.
FIGS. 1 and 2 show an ion source generally at 10 having a cathode
wire 11 and an anode 12. The anode 12 is an annulus having an inner
surface 35 sloping outwards in the direction of the cathode.
Between the cathode 11 and the anode 12 is an ionization region 13.
The cathode wire 11 is suspended above the anode by two mounting
pins 20 that are held by, and in electric isolation from, a shield
plate 30. The shield plate 30 substantially surrounds the anode,
cathode and ionization region by extending from a point lower than
the anode 12 to a point above the cathode 11 and is preferably
maintained at earth potential to shield the anode and the cathode
from external fields. A magnet 14 is disposed outside the
ionization region 13 but adjacent the anode 12. The magnet 14
creates a magnetic field, the longitudinal axis of which is aligned
with the axis of the anode 12. The magnet may be a permanent magnet
or an electromagnet. Preferably the magnet is a high flux rare
earth magnet such as a NdFeB magnet. As an alternative, magnet 14
may be a ring magnet disposed around the anode 12 and ionization
region 13.
The alignment of the magnetic field with the electric field causes
electrons emitted by the cathode to approximately follow the
magnetic field lines as they move towards the anode. This has the
effect of concentrating the flow of electrons toward the axis of
the magnetic field. Therefore the region where the magnetic field
intensity is a maximum, will also be a region of maximum electron
flux.
The ionizable gas, for example oxygen, nitrogen or argon, is
supplied to the ionization region through a gas flow path from gas
feed line 22. The gas flow path terminates at an outlet member 15.
The outlet member 15 has the form of a gas shower head, with a
plurality of apertures 17, that introduce the gas into the
ionization region 13 in a substantially random direction. The gas
shower head 15 is disposed on the axis of the anode and adjacent
the ionization region 13 such that gas emanating from the apertures
17 enters the ionization region at a point of high electron flux.
Because a large proportion of ionization occurs close to the
outlet, the gas shower head is of a material such as stainless
steel, that withstands the very high energy from the incoming
electron flux.
The anode 12 preferably has disposed within it a channel 53 in
communication with a fluid conduit 55 that provides water to cool
the anode. The channel 53 preferably extends into the body of the
outlet member 15.
The anode 12, outlet member 15 and shield 30 are mounted on a non
conductive mounting base 50 through which extends the gas flow path
and fluid conduit 55. A plurality of mounting screws 57 fix the
anode 12 to the base 50. The magnet 14 is housed within the base
such that the external pole is exposed. The mounting base 50 has a
conduit 58 that forms part of the gas flow path and connects the
gas feed line 22 to the outlet member 15 such that no electrical
connection can be made between the outlet member 15 and the gas
feed line 22. The mounting base 50 has a similar conduit for
connecting the water feed line 55 to the channel 53. The gas and
water feed lines preferably screw into the mounting base 50. A
suitable material for the mounting base 50 is glass filled
polytetrafluoroethylene. This arrangement reduces electrical
hazards, simplifies mounting and installation and reduces risk of
secondary plasmas forming within the gas feed line.
The size of the outlet is preferably half or less than the smallest
inner diameter of the anode in order that a localised high pressure
zone is created around the outlet, that decreases rapidly with
distance.
In operation the anode is charged in the range 0-500 V, preferably
250 V relative to the cathode which is at or near earth potential.
A DC current of approximately 12 A is passed through the cathode to
stimulate electron emission. An AC current may be used but the
combination of an alternating current and the magnetic field has
been found by the present inventor to cause vibrations in the
cathode which reduces the cathode lifetime. Electrons generated at
the cathode are influenced by the anode potential and are
accelerated toward it. The magnetic field imparts a spiral motion
on the electrons further increasing their kinetic energy and thus
their potential to ionize gas molecules, and focussing the
electrons toward the longitudinal axis. Collisions between the
energetic electrons with gas molecules emitted from the outlet
member 15 cause ionization. If sufficient ionizing collisions occur
then a plasma is formed. Positive ions created in the plasma
experience the opposite effect to the electrons. The ions initially
have a random velocity but are influenced by the electric potential
gradient which accelerates them toward and past the cathode 11. The
magnetic field in this case acts to control the direction in which
the ions are expelled from the ion source by focusing them into an
ion beam or ion current centred on the longitudinal axis of the
magnetic field. By properly aligning the axis of the magnetic
field, the ion beam can be directed toward a target. Further
features of the above described ion source can be obtained from the
present applicant's co-pending application no. PCT/AU99/0059 1, the
contents of which are incorporated herein by reference.
In a first embodiment, the cathode filament 11 is connected to a DC
power supply that incorporates a variable waveform signal
generator. The cathode filament has a DC biased current of 8-9 A
which is at least 70% and preferably 75-95% of the threshold
current required for thermionic emission (shown in broken line in
FIG. 3). This current is supplied to the filament as a continuous
wave (CW) signal (FIG. 3a). Superimposed on the base signal is a
square pulse signal of approx 3 A (FIG. 3b). The combined signal
(FIG. 3c) is sufficient to produce thermionic emission during peak
periods of the supplementary pulse signal. Because electron
emission from the cathode, and subsequently gas ionization, only
occurs when there is sufficient cathode current, the ion source
will only produce an ion current during peak times of the cathode
current cycle. When the pulse signal finishes and the total
filament current returns to the base level there is no thermionic
electron emission to excite ionization of the gas and thus the ion
beam current falls to zero.
The size of the basic filament signal and the pulse signal can be
varied by the signal generator depending upon requirements and the
type of cathode filament employed, as can the duty cycle and
frequency of the pulse signal.
The present inventor has found it suitable to bombard the target
with ions for between 0.5 and 5 seconds, preferably 1 second, for
every 5 to 30 nm, preferably 10-20 nm of deposition, which
corresponds approximately to having a cathode filament pulse signal
period of approx 5 to 50 seconds, preferably 10 seconds, with duty
cycle of 5% to 30%, preferably 10%. With these parameters, the ion
beam current grows and decays on a time scale much, shorter than
the length of time for which the ion beam current is on during each
cycle.
By providing the filament with a CW base current to which is added
a periodic pulse current, as opposed to providing one pulse signal
of greater amplitude, thermal shock of the filament is prevented
thereby increasing filament lifetime.
An alternative embodiment for producing an intermittent ion beam
current is described with reference to FIG. 4. In this embodiment,
it is the flow of gas into the ionization region that is cyclic.
FIG. 4 shows a gas outlet 15 that is disposed adjacent the
ionization region of the ion source. The outlet is supplied with
gas through a gas feed line 22.
An electrically controlled valve 43 within the gas flow path 22 is
provided with a signal from the signal generator 45 to control its
opening and closing thereby governing the flow of gas into the
ionization region 13.
The waveform used to control the operation of the valve can be the
same square pulse wave used in the cathode filament embodiment
discussed above. However, the rise time of the ion beam current and
its subsequent decay will be longer than for the embodiment of the
cyclic filament current discussed above due to the lag in gas
entering the ionization region once the valve 43 is open and
residual diffusion of gas from the gas path downstream of the valve
43 once the valve is closed. Therefore the pulse which is used to
keep the control valve 43 open may need to be slightly longer than
the analogous filament current pulse in order to provide the same
effective ion beam current intensity and duration. It is also
important that the valve 43 be placed as close to the outlet as
possible to reduce the amount of residual gas in the feed line
downstream of the valve once the valve is closed.
One proposed embodiment for the valve is discussed with reference
to FIG. 5 in which a solenoid coil 60 is wrapped around the gas
feed line 22. An armature 61 disposed within the gas line 22 in a
first position engages a valve seat 63 to close the valve and in a
second position allows gas to flow through the line to the outlet.
The armature 61 is ferromagnetic and/or paramagnetic and in normal
conditions will be biased, either by a spring 65 or by the magnetic
field of the rare earth magnet 14 used for controlling expulsion of
ions from the plasma, to close the valve. A change in the electric
current in the solenoid 60 as governed by the waveform from the
signal generator 45 creates a change in the local magnetic
conditions around the armature 61. This has the effect of drawing
the armature 61 away from the valve seat 63 thereby allowing gas
flow. A reverse change in the charging state of the solenoid causes
the armature 61 to re-engage the valve seat 63.
It is preferred that the armature 61 be magnetised such that in the
absence of the solenoid field, the armature 61 is repelled by the
rare earth magnet 14 to the closed position. When the solenoid is
charged the solenoid field overcomes the field from the magnet 14
and acts to open the valve. In this way, the gas line remains
closed in the absence of any power.
In an alternative system, the valve may be a piezo-valve opened and
closed as the governing signal from the waveform generator goes
between high and low values.
In a further embodiment, one or more gas injectors can be used in
place of the gas outlet and valve. The gas injectors inject a
measured volume of gas into the ionization region. The timing of
the gas injection can be controlled by the waveform generator. An
advantage of periodic or cyclic gas flow is that the overall amount
of gas provided to the ionization region can be reduced because no
gas is provided during times when no ion current is required, and
thus smaller and/or cheaper vacuum pumps can be used to achieve the
low pressures required for optimum IAD conditions.
A more significant advantage is that with pulsed gas flow it is
possible to achieve higher localised gas pressures in the
ionization region whilst still maintaining a low background
pressure for the same pump system. This is because the average gas
flow and thus the average background pressure over a number of
cycles is lower for pulsed gas flow than for continuous gas flow
conditions using the same gas flow rates. The system can thus
tolerate higher gas flow rates during the on-phase of the gas flow
cycle without introducing instabilities into the system that are
caused by high background pressures. The higher gas flows create
higher ionization region. pressures giving rise to higher ion beam
currents.
A further method for creating a pulsed ion beam current is to
provide the ion source anode 12 with a square pulse waveform
voltage ranging from 0 V to its normal operating voltage eg 250 V,
relative to the cathode. When the anode voltage is high, the ion
source operates as normal to produce an ion beam current. When the
anode voltage goes low, electrons created at the cathode are not
influenced by the anode voltage and thus do not preferentially
accelerate toward the ionization region. Thus minimal excitation
collisions occur and no meaningful ion beam is produced.
Further methods for producing a cyclic ion beam may be possible.
For example, in ion sources where an electromagnet is used to shape
and direct the ion beam, the electromagnet may receive a pulsed
signal so that the directional effects that cause the ions to flow
toward the target are produced only intermittently.
It is of course possible to combine any of the above described
methods to produce an intermittent ion beam current.
Referring to FIG. 6 there is schematically shown an ion assisted
thin film deposition system including an ion source 70 according to
the invention, deposition apparatus 71 and a target substrate 72
all disposed within a vacuum chamber 73. The power supply 74
operating the ion source includes a waveform generator 45 used to
produce a periodic ion beam 80 directed toward the target substrate
72. The deposition apparatus 71 produces the vapour stream of
material 79 to be deposited and may employ any known technique used
in physical vapour deposition, such as thermal evaporation or
electron beam evaporation. The specific vaporisation techniques
used will depend on the type of material being deposited and the
substrate type.
In operation, a pre-determined thickness of material is deposited,
without ion assistance, which results in the growth of a
stoichiometric deposit with low packing density. The deposited film
is then bombarded with a short-duration high-energy pulse of ions.
This procedure is repeated until the full thickness of film is
achieved. The result is a strongly adherent, stoichiometric film
with good bulk density and optical properties.
In a most preferred form of the invention, deposition of the
coating material onto the target 72 also occurs cyclically with
each of the stages of deposition and ion bombardment occurring
non-concurrently, that is, to the exclusion of the other stage. The
waveform generator 45 can be used to control the vaporisation
apparatus 71 with a signal the temporal inverse of the signal used
to control the ion source. The target 72 thus receives a repeated
cycle consisting of exclusive and distinct deposition and ion
bombardment stages.
In an alternative form, the vaporisation apparatus may have a
shutter member 78 that is actuated at the start of the ion
bombardment phase of the total process cycle to block the stream of
deposition material 79 to the substrate so that further deposition
material is prevented from reaching the target whilst ion
bombardment is occurring. The shutter can be controlled by the same
waveform from the waveform generator that controls the ion source.
On completion of the ion bombardment phase the shutter is moved out
of the stream of deposition material.
Instead of using the waveform generator to trigger production of
the ion beam current, the ion source may use an external trigger.
For example, the ion source may receive feedback from a deposition
monitor 76, e.g. a quartz crystal monitor, that measures growth of
the deposited film in situ. Once a predetermined thickness of film
has been deposited a control signal may be generated to trigger the
ion source to provide a pulse of ions causing densification of the
most recently deposited film. The pulse size and duration may still
be governed by the waveform generator or by other means. The same
trigger used to activate the ion beam may control the vaporisation
apparatus and/or shutter member to prevent further film material
from being deposited on the substrate during ion bombardment.
The duration of the ion bombardment phase is predetermined by the
pulse length setting on the waveform generator.
At the conclusion of the ion bombardment phase, the pressure may be
higher than is required or desirable for the deposition phase due
to the injection of gas into the ion source. A pressure transducer
75 disposed within the vacuum chamber can measure the chamber
pressure which the system can use to prevent the deposition process
from recommencing until the pressure is below a predetermined
level.
Case Study 1: Magnesium Fluoride
Magnesium fluoride is a very commonly used thin film material for
application to both single and multi-layer antireflection coatings.
It possesses a low refractive index (n=1.35 at 550 nm) and a
transparency range from the deep ultraviolet to the far infrared.
Conventional deposition requires high substrate temperatures
(=300.degree. C.) which can increase processing time in multi-stage
processes and considerably, increase the risk of damaging thermally
sensitive substrates. Comparative results for deposition of
magnesium fluoride with continuous and periodic ion bombardment of
the target are shown in Table 1 below.
TABLE 1 Method of Deposition Properties n @550 nm
Evaporated-unheated Soft, easily damaged low 1.35 substrates
packing density, high stress, unstable Evaporated-hot 300.degree.
C. More durable, n increasing 1.39 typical IAD O+, cold Dense, n
increasing 1.40-1.43 IAD Ar+, cold Dense, k increasing 1.40 PULSED
O+ IAD* Very durable, High 1.35 (bulk) Unheated substrates
transparency *Ion assisted with 250 eV oxygen ion energy, 750 mA
pulses with 10% duty cycle
Case Study 2. Calcium Fluoride
Bulk calcium fluoride possesses the lowest refractive index of any
thin film material with an index of n=1.21 (bulk) at 550 nm. The
material has a very wide transparency range comparable to magnesium
fluoride above. Evaporated CaF.sub.2 thin films have a packing
density of only 50%-60% and are thus extremely soft and easily
damaged, making it almost essential that they are used in a clean
environment as wiping will quickly damage the coating. Table 2
shows comparative data for calcium fluoride films deposited with
and without pulsed ion bombardment.
TABLE 2 Method of Deposition Properties n @550 nm Thermally
evaporated- Very fragile and unstable, =1.20 in vacuum cold large
vacuum to air shift, =1.28-1.30 in air uncleanable. PULSED-O+ IAD*
Soft but stable films =1.22 to 1.23 Unheated substrates negligible
vacuum to air shift. Cleanable. *Ion assisted with 200 eV oxygen
ion energy, 500 mA pulses with 8% duty cycle
Because the ion source of the present invention operates in a
cyclic mode so that the ion beam is only produced for brief
periods, instabilities that grow within the ion source during the
on-phase may not be fatal to the ion source's operation if the ion
source switches to the off phase before a catastrophic event, such
as the development of a vacuum arc, occurs. For example, it may be
possible to have a higher gas flow rate during the on-phase of a
pulsed ion beam system than for a continuous system, giving rise to
a higher ion beam current, because by the time the pressure outside
the ionization region reaches the levels where vacuum arcs may
occur, the waveform signal will go low thus switching off the gas
flow. The potential instability will then stabilise before the next
on-phase of the cycle commences.
Using an intermittent ion beam in ion assisted deposition of films
prevents or at least reduces the problems discussed above of prior
art IAD systems such as ion species depletion and displacement.
This is because minimal new material is deposited during the ion
bombardment phase of the cycle. Thus the ion beam serves only as a
source of energy for densifying the already deposited material. The
problems of the prior art can be further reduced by excluding
deposition totally during the ion bombardment phase.
The present invention has further enabled the production of stable,
optical quality UV films.
While particular embodiments of this invention have been described,
it will be evident to those skilled in the art that the present
invention may be embodied in other specific forms without departing
from the essential characteristics thereof. The present embodiments
and examples are therefore to be considered in all respects as
illustrative and not restrictive, the scope of the invention being
indicated by the appended claims rather than the foregoing
description, and all changes which come within the meaning and
range of equivalency of the claims are therefore intended to be
embraced therein. It will further be understood that any reference
herein to known prior art does not, unless the contrary indication
appears, constitute an admission that such prior art is commonly
known by those skilled in the art to which the invention
relates.
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